It is now well known to electrolyze brine and other halides in the electrolytic cells containing anode and cathode compartments separated by a liquid and gas impervious permselective membrane. The voltages required for electrolysis of halides in such membrane cells are, however, relatively high; one of the reasons being that the electrochemically active anode and cathode surfaces are physically separated from the permselective membrane. This introduces IR drops due to the layers of electrolyte between the membrane and electrodes and IR drops due to gas blinding effects caused by bubbles or films of chlorine and hydrogen between the electrochemically active gas evolving electrode surfaces and the membrane. In a recently filed co-pending application Ser. No. 922,316, entitled Production of Halogens by Electrolysis of Alkali Metal Halides in an Electrolysis Cell Having Catalytic Electrodes Bonded to the Surface of a Solid Polymer Electrolyte Membrane, filed July 6, 1978, in the name of A. B. LaConti, et al, assigned to The General Electric Company, the assignee of the present invention, a process for producing alkali metal hydroxide and halogens is described in which the electrochemically active anode and cathode electrodes, in the form of porous, bonded masses of electrocatalytic and polymeric particles, are bonded directly to and embedded in the permselective membrane to form a unitary membrane-electrode structure. Substantial reductions in cell voltages are realized since the reaction sites are moved directly to the membrane and electrolysis takes place essentially at the interface of the bonded electrode and the membrane so that electrolyte IR drops and IR drops due to gas blinding effects are minimized. Current flow to the bonded embedded anode and cathode electrodes is by means of separate current collectors pressed against the bonded electrodes and connected to the cell power supply.
Good contact must be maintained between the current collectors and the electrodes in order to minimize ohmic losses and associated voltage drops at the collector/electrode interface. In the aforesaid LaConti application and cells of the type having electrode bonded directly to the surface of the membrane, the current collectors are clamped between the housing and membrane by mechanical, hydraulic or other clamping mechanisms to maintain good contact between the current collector and the electrode.
In the application (52-EE-0-327) Ser. No. 038,812 Coker, et al, filed May 14, 1979, concurrently with the instant application and assigned to The General Electric Company the assignee of the present invention, a process and electrolytic cell is described in which excellent electrode/current collector contact is maintained by utilizing a three compartment cell in which the buffer (or central) compartment is operated at a positive pressure with respect to the other compartments by supplying a pressurized feed to the buffer compartment. This forces the unitary membrane/electrode structure outward against the current collectors establishing uniform, constant and controllable contact pressure.
Applicants have found that it is possible to insure good electrode/current collector contact in a three chamber electrolyzer by operating the buffer compartment at a positive pressure differential generated solely by mass transport of H.sub.2 O across the membrane. This results in self-pressurization of the buffer compartment thereby eliminating pressurizing equipment for the buffer compartment feed, i.e., pumps, filter, conduits, tanks, etc.
This is readily achieved by utilizing anode and cathode permselective membranes with controlled water transport characteristics. The membranes are chosen so that transport of water into the buffer compartment as ions move through the membranes is greater than that out of the buffer compartment. Pressurization of the buffer compartment is thus accomplished solely by passing current through the cell.
Use of a self-pressurized buffer compartment not only results in lower cell voltages but also increases the cathode current efficiency by discharging back migrating hydroxyl ions from the buffer compartment as sodium hydroxide. Current efficiencies of 90% or more at high caustic concentrations (5 M or higher) are readily achievable by incorporating multiple hydroxide rejection stages in a single cell process.
It is therefore, a principal objective of this invention to provide a three compartment electrolytic cell and a process for generating halogens and alkali metal hydroxide therein using a self-pressurized buffer compartment to maintain positive buffer compartment pressures with respect to the other compartments.
Another objection of the invention is to provide a three compartment electrolytic cell and an electrolysis process to be carried out therein in which the buffer compartment is self-pressurized to maintain uniform, constant, and controllable contact between the electrochemically electrodes physically bonded to the permselective membrane and the associated current collectors associated therewith.
Still another objective of the invention is to provide a three compartment electrolytic cell in which the buffer compartment is self-pressurized solely by passing current through the cell.
Yet another objective of the invention is to provide a highly efficient three compartment electrolytic cell and a process for generating chlorine and caustic therein in which the cell electrolysis voltage is minimized by maintaining uniform, constant and controllable contact pressure between the cell membranes by means of a buffer compartment which is self-pressurized to operate at a positive pressure with respect to the other compartments.
Other objectives and advantages of the invention will become apparent as the description thereof proceeds.
The objectives and advantages of the invention are realized in an electrolytic cell having a pair of liquid and gas impervious permselective membranes, which divide the cell into anode, cathode, and buffer compartments. Electrodes in the form of bonded masses of polymeric and electrochemically active particles are bonded to membranes and face the anode and cathode compartments. Separate current collectors are positioned in physical contact with the electrochemically active electrodes and connect them to a source of electrolyzing voltage. The two membranes have transport characteristics such that more water is carried by the ions into the buffer compartment than is carried out of it, so that pressure is built up rapidly in the buffer compartment by normal operation of the cell; i.e., by driving current through the cell. The positive pressure forces the membrane outward into firm contact with the current collectors thereby maintaining the desired uniform, constant contact pressure. By maintaining a positive pressure differential of at least 0.5 psi and up to 5 psi; and preferably in the range of 1-2 psi cell voltages in the range of 3.35 to 3.55 volts at 300 ASF are readily achievable and represent voltage improvements ranging from 0.6 to 1.5 volts over conventional three compartment cells at 300 ASF. The cell pressure is maintained at the desired value by pressure relief valves or standpipes associated with the buffer compartment.